1. Field of the Invention
The present invention relates to a diode detecting circuit for detecting an electromagnetic wave signal.
2. Description of the Related Art
A radio communication apparatus for detecting an electromagnetic wave signal of a particular frequency use a receiving apparatus. In order to start the receiving operation, the receiving apparatus determines whether the radio signal of the particular frequency to be received exists. In recent years, such radio communication apparatuses are typically required to have the capability of detecting a faint radio signal. Moreover, a need has arisen to provide a detecting circuit for such apparatuses that has a higher sensitivity, but that is available at a low price and has low power consumption.
FIG. 1 is a circuit diagram illustrating a related art diode detecting circuit 10, of the half-wave rectifying type, for detecting a low level signal with high accuracy. The diode detecting circuit 10 of FIG. 1 has a capacitor 11, an inductor 12, a diode 13, a capacitor 14, a resistor 15, a constant voltage source 16, an input terminal 17 and an output terminal 18. An input voltage signal VIN is applied to the input terminal 17. The capacitor 11 shifts the input voltage signal VIN via capacitance coupling, and outputs an alternating input voltage signal VAC to node A. In addition, node A is coupled to a bias voltage VOFFSET of the constant voltage source 16 via an inductor 12. Due to the presence of the inductor 12, the voltage at node A varies freely in response to an applied high frequency voltage variation, whereas the voltage at node A is fixed at the bias voltage VOFFSET for low frequency voltage variation. Accordingly, the alternating input voltage signal VAC is biased by the bias voltage VOFFSET at node A, i.e., a diode input voltage signal VDIN which is applied to an anode terminal of the diode 13 equals the alternating input voltage signal VAC plus the bias voltage VOFFSET.
As further shown in FIG. 1, when the diode input voltage signal VDIN exceeds the threshold voltage of the diode 13, a current signal IDO flows through the diode 13 in the forward direction, in accordance with the voltage of the diode input voltage signal VDIN at node A. Since the current signal IDO flows through the resistor 15 connected to a cathode terminal of the diode 13, the current signal IDO is converted to a voltage signal. In addition, this voltage signal is smoothed by the capacitor 14 connected in parallel to the resistor 15. Consequently, an output voltage VOUT, which was smoothed by the capacitor 14, appears at the output terminal 18.
In the diode detecting circuit 10 of FIG. 1, the center voltage in the alternating diode input voltage signal VDIN, which is applied to the anode terminal of the diode 13, can be set to a voltage near the threshold voltage of the diode 13 by adjusting the bias voltage VOFFSET of the constant voltage source 16. Accordingly, a low-level input voltage signal can be detected with high accuracy without the need to use an expensive diode having a low threshold voltage as the diode 13.
However, the threshold voltage of the diode 13 of FIG. 1 varies depending on temperature. Therefore, when temperature varies, the threshold voltage of diode 13 varies and deviates from the center voltage, which is typically set to the bias voltage VOFFSET. In this situation, a low-level input voltage signal VIN cannot be detected with high accuracy.
FIG. 2 is a circuit diagram illustrating a diode detecting circuit 20, which is disclosed, for example, in Japanese Patent Application Publication No. 07-111421. The diode detecting circuit 20 of FIG. 2 eliminates the influence of temperature on circuit performance.
A diode detecting circuit 20 of FIG. 2 has a diode 21, a resistor 22, a differential amplifier 23, a feedback diode 24, and a resistor 25. An input voltage signal applied to an input terminal IN is rectified and detected by the diode 21 and resistor 22, and is then provided to a non-inverted input terminal (+) of the differential amplifier 23. An inverted input terminal (−) of the differential amplifier 23 is coupled with the ground potential via the resistor 25, and an output terminal of the differential amplifier 23 is coupled with the inverted input terminal (−) of the differential amplifier 23 through a feedback diode 24. The diode 21 and the feedback diode 24 have similar characteristics. Therefore, temperature dependence of the threshold voltage of the diode 21 that appears in the voltage signal supplied to the non-inverted input terminal (+) of the differential amplifier 23 is canceled by temperature dependence of the threshold voltage of the feedback diode 24 that appears in the signal supplied to the inverted input terminal (−) of the differential amplifier 23. Accordingly, the detection accuracy of the diode detecting circuit 20 of FIG. 2 is free from the influence of temperature effects on the diode 21.
However, the diode detecting circuit 20 of FIG. 2 has a problem in that bias voltages cannot be applied to the diodes 21 and 24. Namely, if a bias voltage is applied to the anode terminal of the diode 21, the same bias voltage should be applied to the anode terminal of the feedback diode 24 to compensate for the temperature dependence in the threshold voltage of the diode 21. In this situation, since the anode terminal of diode 24 is connected to the output terminal of the differential amplifier 23, current from a bias voltage source flows into the differential amplifier 23 through the output terminal of the differential amplifier 23, and an adequate bias voltage cannot be set. Accordingly, in the diode detecting circuit 20 of FIG. 2, bias voltages cannot be applied to the diodes 21 and 24. Therefore, expensive diodes having low threshold voltages must be used as the diodes 21 and 24, to allow detection of low-level input voltage signal with high accuracy.
Moreover, since it is generally difficult to form a diode having a low threshold voltage within an integrated circuit (IC) chip, individual diodes must be used to produce a diode detecting circuit using an IC chip.
There remains an unmet need in the related art to provide a diode detecting circuit which is stable under variation of temperature, without using an expensive diode having a low threshold voltage.